Use data not wifi for backup

Backup to iCloud: Wifi connection is mandatory.

Backup to a computer using iTunes: No need to use wifi connection or cellular data. Your device should be connected to a computer's USB port.

Regards

Giulio

You can:

borrow a computer running iTunes from your friends

or

make an appointmnent at an Apple Store or Authorized Service Provider and ask for help.

Regards

Giulio